1. Field of the Invention
The present invention relates to an apparatus for optically detecting the surface position of an object. More particulary the invention relates to an apparatus so designed that in the focusing mechanism or the leveling mechanism of a projection exposure apparatus, the detection is made of the position level or the inclination of a surface of an object in an exposing area or the like defined by the projection exposure apparatus.
2. Description of the Prior Art
With projection exposure apparatus of the type heretofore used for the manufacture of VLSI's, particularly semiconductor devices having very fine structures, each of an automatic focusing and an automatic leveling constitutes one of important mechanisms for effecting the proper exposure. The surface of a wafer must be accurately arranged by these mechanisms at a focal position of a projection optical system or a position vertical to the optical axis of the projection optical system.
Detecting apparatus heretofore known for obtaining the position information of a wafer surface for such focusing mechanism and leveling mechanism have been disclosed for example in the specification of U.S. Pat. No. 4,558,949 assigned to the same assignee as the present application. This detecting apparatus utilizes an oblique illumination system in which a detecting light is introduced into a space between a projection optical system and the surface of a wafer. That is, the light is supplied from an oblique direction with respect to the optical axis of a projection objective lens so that the light is reflected by the wafer surface. The position information of the wafer surface is obtained from the reflected light. The position of the wafer surface is adjusted by the focusing mechanism and the leveling mechanism in accordance with the thus obtained information.
With the conventional position detecting apparatus of the above-mentioned oblique projection type, it has been necessary to ensure that the space between a projection optical system and the surface of a wafer or the working distance of the projection optical system is sufficient. This is not only extremely disadvantageous from the standpoint of the aberration compensation of the projection optical system but also causes a heavy burden from the lens manufacturing point of view.
For instance, in the projection optical system there is the need on the wafer side to prevent as far as possible the occurrence of aberrations through the arrangement for example of a lens which is nearly aplanatic from the aberration compensation point of view. Thus, in view of the recent demand for increasing the numerical aperture (N.A.) of the projection optical system and also increasing the field, the apertures of its lenses tend to be increased more and more. If the working distance of the projection optical system is increased under such conditions for the purpose of arranging a position detecting optical system of the oblique projection type, the lenses of the projection optical system tend to be increased in aperture still more. As a result, there is the disadvantage that increasing the lens apertures in the projection optical system which is nearly aplanatic not only causes more difficulties in the lens manufacture but also tends to make its optical performance dependent on a distortion due to its own weight.